As far as windows and ram requirements, it is possible to install windows 7 on very low-end systems (even 128mb of ram) with some tricks. But aside from the novelty the utility is questionable. A 512mb stick of PC133 (the functional minimum for a Windows 7 32-bit installation) costs a mere $10 or less, which is a tiny fraction of the cost of buying Windows 7 itself. Setting the minimum requirement that "high" saves a good amount on testing, and it also ensures that customers don't end up with extremely underpowered machines (a problem that plagued the Windows Vista launch and led to law suits).

It's not entirely difficult to shoehorn Windows 7 onto such systems either, I suspect it's sort of designed that way, to ensure that the only people who do it are the people who know what they're doing and won't be disappointed by the results.